The invention generally relates to switching operation of a fuel cell and more particularly, the invention generally relates to automatically switching between a mode in which the fuel cell provides power and a mode in which the fuel cell functions as an electrochemical pump.
A fuel cell is an electrochemical device that converts chemical energy directly into electrical energy. For example, one type of fuel cell includes a proton exchange membrane (PEM) that permits only protons to pass between an anode and a cathode of the fuel cell. Typically PEM fuel cells employ sulfonic-acid-based ionomers, such as Nafion, and operate in the 60° Celsius (C.) to 70° temperature range. Another type employs a phosphoric-acid-based polybenziamidazole, PBI, membrane that operates in the 150° to 200° temperature range. At the anode, diatomic hydrogen (a fuel) is reacted to produce hydrogen protons that pass through the PEM. The electrons produced by this reaction travel through circuitry that is external to the fuel cell to form an electrical current. At the cathode, oxygen is reduced and reacts with the hydrogen protons to form water. The anodic and cathodic reactions are described by the following equations:H2→2H++2e− at the anode of the cell, and   Equation 1O2+4H++4e−→2H2O at the cathode of the cell.   Equation 2
A typical fuel cell has a terminal voltage near one volt DC. For purposes of producing much larger voltages, several fuel cells may be assembled together to form an arrangement called a fuel cell stack, an arrangement in which the fuel cells are electrically coupled together in series to form a larger DC voltage (a voltage near 100 volts DC, for example) and to provide more power.
The fuel cell stack may include flow plates (graphite composite or metal plates, as examples) that are stacked one on top of the other, and each plate may be associated with more than one fuel cell of the stack. The plates may include various surface flow channels and orifices to, as examples, route the reactants and products through the fuel cell stack. Several PEMs (each one being associated with a particular fuel cell) may be dispersed throughout the stack between the anodes and cathodes of the different fuel cells. Electrically conductive gas diffusion layers (GDLs) may be located on each side of each PEM to form the anode and cathodes of each fuel cell. In this manner, reactant gases from each side of the PEM may leave the flow channels and diffuse through the GDLs to reach the PEM.
The fuel cell stack is one out of many components of a typical fuel cell system, such as a cooling subsystem, a cell voltage monitoring subsystem, a control subsystem, a power conditioning subsystem, etc. The particular design of each of these subsystems is a function of the application that the fuel cell system serves.
The membranes of a newly assembled fuel cell stack typically are conditioned by cycling the membranes through an incubation period, a period of stack operation to “break-in” the membranes. Until the membranes are broken in, the terminal voltage of the stack gradually rises over time before the terminal voltage stabilizes near a generally constant voltage level to mark the end of the incubation period. Among the possible theories to explain why the incubation period is needed, the membranes may include catalyst residue that, until removed during the incubation period, hinders the performance of the membranes. Another theory is that the membranes are initially dry, a condition that hinders the performance of the stack until the membranes hydrate during the incubation period.
The conditioning of the fuel cell stack is a lengthy process that may involve a considerable amount of time changing electrical and plumbing connections. Thus, there exists a continuing need for better ways to condition a fuel cell stack.